Various electromagnetic techniques exist to perform surveys of subterranean structures underneath a surface for identifying structures of interest. Examples of structures of interest in the subterranean structure include subsurface resistive bodies, such as oil-bearing reservoirs, gas injection zones, and fresh-water aquifers. One survey technique is the magnetotelluric (MT) survey technique that employs time measurements of electric and magnetic fields (which are responsive to naturally occurring electromagnetic fields) for determining the electrical conductivity distribution beneath the surface. Another survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic transmitter is used to generate electromagnetic signals. With either survey technique, surveying units (or receivers) containing electric and magnetic field sensors are deployed on a surface within an area of interest to make measurements from which a geological survey of the subterranean structure underneath the surface can be derived.
Forward modeling is often performed to develop models for modeling the EM response in a subterranean structure. Typically, the forward modeling involves producing a mathematical model from which synthetic (simulated) EM data can be calculated. The synthetic EM data is then compared to real EM data, and if the synthetic data and EM data do not match to within predefined convergence criteria, the model is altered and the above process is repeated iteratively until a satisfactory match is obtained between the synthetic EM data and the real EM data.
Typically, a forward model is constructed for a relatively large frequency interval (containing multiple frequencies) and a relatively large time domain (including many time points). This results in relatively large forward models that are computationally expensive to compute and to use.